Recently, carbon-neutral biofuels are attracting attention as a global warming countermeasure. Among such biofuels, bioethanol is particularly attracting attention. Bioethanol is sometimes used directly as an alternative for gasoline, but is usually used by being mixed with petroleum gasoline. However, there is one drawback, in that phase separation easily happens when ethanol and gasoline are mixed. A solution for this is the use of alcohols such as butanol, which have larger carbon numbers than ethanol and which can be easily mixed with gasoline (being resistant to phase separation). But methods for obtaining butanol from biological resources are limited. In other words, methods for obtaining ethanol comprising fermenting a starch from sources such as corn and potatoes or sugar obtained from a source such as sugarcane by using enzymes have been established to a certain degree, but enzymes for efficiently producing butanol by fermentation have been unknown.
In view of the above, the present invention relates to a technique that was developed based on the conception of obtaining biobutanol by conversion of the obtained bioethanol to butanol by using a catalyst. On the other hand, there is a similar technique for obtaining acetaldehyde, butadiene, 1-butanol, high-octane fuels, and mixtures thereof from ethanol by a gas-phase reaction process using calcium phosphate apatite (Patent Documents 1 and 2).
However, even when these methods are applied to such purpose as above, the butanol selectivity is still unsatisfactory for industrialization purposes and a method for synthesizing butanol from ethanol at a higher selectivity was desired.    [Patent document 1] Japanese Laid-Open Patent Application No. 11-217343    [Patent document 2] WO2006/059729A1